The <xsl:strip-space> and <xsl:preserve-space>
elements are used to control the way whitespace nodes in the source XML
document are handled. These elements have no impact on whitespace in the XSLT
stylesheet. Both elements can occur only as top-level elements, possible more
than once, and the elements are always empty

Both elements take one attribute "elements" which contains a
whitespace separated list of named nodes which should be or preserved
stripped from the source document. These names can be on one of these three
formats (NameTest format):

All whitespace nodes:
elements="*"

All whitespace nodes with a namespace:
elements="<namespace>:*"

Specific whitespace nodes: elements="<qname>"

Identifying strippable whitespace nodes

The DOM detects all text nodes and assigns them the type TEXT.
All text nodes are scanned to detect whitespace-only nodes. A text-node is
considered a whitespace node only if it consist entirely of characters from
the set { 0x09, 0x0a, 0x0d, 0x20 }. The DOM implementation class has a static
method used to detect such nodes:

The DOM has a bit-array that is used to tag text-nodes as strippable
whitespace nodes:

private int[] _whitespace;

There are two methods in the DOM implementation class for accessing this
bit-array: markWhitespace(node) and isWhitespace(node).
The array is resized together with all other arrays in the DOM by the
DOM.resizeArrays() method. The bits in the array are set in the
DOM.maybeCreateTextNode() method. This method must know whether
the current node is a located under an element with an
xml:space="<value>" attribute in the DOM, in which
case it is not a strippable whitespace node.

An auxillary class, WhitespaceHandler, is used for this purpose. The class
works in a way as a stack, where you "push" a new strip/preserve setting
together with the node in which this setting was determined. This means that
for every time the DOM builder encounters an xml:space attribute
it will invoke a method on an instance of the WhitespaceHandler class to
signal that a new preserve/strip setting has been encountered. This is done
in the makeAttributeNode() method. The whitespace handler stores the
new setting and pushes the current element node on its stack. When the
DOM builder closes up an element (in endElement()), it invokes
another method of the whitespace handler to check if the strip/preserve
setting is still valid. If the setting is now invalid (we're closing the
element whose node id is on the top of the stack) the handler inverts the
setting and pops the element node id off the stack. The previous
strip/preserve setting is then valid, and the id of node where this setting
was defined is on the top of the stack.

Determining which nodes to strip

A text node is never stripped unless it contains only whitespace
characters (Unicode characters 0x09, 0x0A, 0x0D and 0x20). Stripping a text
node means that the node disappears from the DOM; so that it is never
included in the output and that it is ignored by all functions such as
count(). A text node is preserved if any of the following apply:

the element name of the parent of the text node is in the set of
elements listed in <xsl:preserve-space>

the text node contains at least one non-whitespace character

an ancenstor of the whitespace text node has an attribute of
xsl:space="preserve", and no close ancestor has and
attribute of xsl:space="default".

Otherwise, the text node is stripped. Initially the set of
whitespace-preserving element names contains all element names, so the
default behaviour is to preserve all whitespace text nodes.

This seems simple enough, but resolving conflicts between matching
<xsl:strip-space> and <xsl:preserve-space>
elements requires a lot of thought. Our first consideration is import
precedence; the match with the highest import precedence is always chosen.
Import precedence is determined by the order in which the compared elements
are visited. (In this case those elements are the top-level
<xsl:strip-space> and <xsl:preserve-space>
elements.) This example is taken from the XSLT recommendation:

stylesheet A imports stylesheets B and C in that order;

stylesheet B imports stylesheet D;

stylesheet C imports stylesheet E.

Then the order of import precedence (lowest first) is D, B, E, C, A.

Our next consideration is the priority of NameTests (XPath spec):

elements="<qname>" has priority 0

elements="<namespace>:*" has priority -0.25

elements="*" has priority -0.5

It is considered an error if the desicion is still ambiguous after this,
and it is up to the implementors to decide what the apropriate action is.

With all this complexity, the normal usage for these elements is quite
smiple; either preserve all whitespace nodes but one type:

<xsl:strip-space elements="foo"/>

or strip all whitespace nodes but one type:

<xsl:strip-space elements="*"/>
<xsl:preserve-space elements="foo"/>

Stripping nodes

The ultimate goal of our design would be to totally screen all stripped
nodes from the translet; to either physically remove them from the DOM or to
make it appear as if they are not there. The first approach will cause
problems in cases where multiple translets access the same DOM. In the future
we wish to let translets run within servlets / JSPs with a common DOM cache.
This DOM cache will keep copies of DOMs in memory to prevent the same XML
file from being downloaded and parsed several times. This is a scenarios we
might see:

Figure 1: Multiple translets accessing a common pool of DOMs

The three translets running on this host access a common pool of 4 DOMs.
The DOMs are accessed through a common DOM interface. Translets accessing
a single DOM will have a DOMAdapter and a single DOMImpl object behind this
interface, while translets accessing several DOMs will be given a MultiDOM
and a set of DOMImpl objects.

The translet to the left may want to strip some nodes from the shared DOM
in the cache, while the other translets may want to preserve all whitespace
nodes. Our initial thought then is to keep the DOM as it is and somehow
screen the left-hand translet of all the whitespace nodes it does not want to
process. There are a few ways in which we can accomplish this:

The translet can, prior to starting to traverse the DOM, send a reference
to the tables containing information on which nodes we want stripped to
the DOM interface. The DOM interface is then responsible for hiding all
stripped whitespace nodes from the iterators and the translet. A problem
with this approach is that we want to omit the DOM interface layer if
the translet is only accessing a single DOM. The DOM interface layer will
only be instanciated by the translet if the stylesheet contained a call
to the document() function.

The translet can provide its iterators with information on which nodes it
does not want to see. The translet is still shielded from unwanted
whitespace nodes, but it has the hassle of passing extra information over
to most iterators it ever instanciates. Note that all iterators do not
need be aware of whitepspace nodes in this case. If you have a look at
the figure again you will see that only the first level iterator (that is
the one closest to the DOM or DOM interface) will have to strip off
whitespace nodes. But, there may be several iterators that operate
directly on the DOM ( invoked by code handling XSL functions such as
count()) and every single one of those will need to be told
which whitespace nodes the translet does not want to see.

The third approach will take advantage of the fact that not all
translets will want to strip whitespace nodes. The most effective way of
removing unwanted whitespace nodes is to do it once and for all, before
the actual traversal of the DOM starts. This can be done by making a
clone of the DOM with exlusive-access rights for this translet only. We
still gain performance from the cache because we do not have to pay the
cost of the delay caused by downloading and parsing the XML source file.
The cost we have to pay is the time needed for the actual cloning and the
extra memory we use.

Normally one would imagine the translet (or the wrapper class that
invokes the translet) calls the DOM cache with just an URL and receives
a reference to an instanciated DOM. The cache will either have built
this DOM on-demand or just passed back a reference to an existing tree.
In this case the DOM would need an extra call that a translet would use
to clone a DOM, passing the existing DOM reference to the cache and
recieving a new reference to the cloned DOM. The translet can then do
whatever it wants with this DOM (the cache need not even keep a reference
to this tree).

We are lucky enough to be able to combine the first two approaches. All
iterators that directly access the DOM (axis iterators) are instanciated by
calls to the DOM interface layer (the DOM class). The actual iterators are
created in the DOM implementation layer (the DOMImpl class). So, we can pass
references to the preserve/strip whitespace tables to the DOM, and the DOM
will make sure that all axis iterators return node sets with respect to these
tables.

Filtering whitespace nodes

For each axis iterator and for DOM.makeStringValue() and
DOM.stringValueAux() we must apply a filter for eliminating all
unwanted whitespace nodes. To achive this we must build a very efficient
predicate for determining if the current node should be stripped or not. This
predicate is built by Whitespace.compilePredicate(). This method is
static and builds a predicate for a vector of WhitespaceRule objects. (The
WhitespaceRule class is defined within the Whitespace class.) Each
WhitespaceRule object contains information for a single element listed
in an <xsl:strip/preserve-space> element, and is broken down
into the following information:

the namespace (can be the default namespace)

the element name or "*"

the type of rule; NS:EL, NS:* or *

the priority of the rule (based on import precedence and type)

the action; either strip or preserver

The Vector of WhitespaceRules is arranged in order of priority and
redundant rules are removed. A predicate method is then compiled into the
translet as:

public boolean stripSpace(int node);

Unfortunately this method cannot be declared static.

When the Stylesheet objectcompiles the topLevel() method of the
translet it checks for the existance of the stripSpace() method. If
this method exists the topLevel() will be compiled to pass the
translet to the DOM as a StripWhitespaceFilter (the translet implements this
interface when the stripSpace() method is compiled).

All axis iterators and the DOM.makeStringValue() and
DOM.stringValueAux() methods check for the existance of this filter
(it is kept in a global variable in the DOM implementation class) and takes
the appropriate actions. The methods in the DOM for returning axis iterators
will place a StrippingIterator on top of the axis iterator if the filter is
present, and the two methods just mentioned will return empty strings for
whitespace nodes that should be stripped.